1. Field of the Invention
This invention relates generally to excimer or "excited dimer" lasers; and more particularly to such lasers that are excited longitudinally by electrical discharges.
2. Prior Art
In recent years the excimer laser has moved swiftly from laboratory experimentation to a commercial product applied in many industrial fields. This development is due to the deep-ultraviolet operating range and associated high photon energy of the excimer laser, and to its high gain, short pulse duration, and freedom from speckle.
Excimers are gas-molecular species that exist only in excited states. Thus the excimer is created and pumped (elevated in energy state) simultaneously, although some further pumping can occur after the species is created. Excitation is provided by an electrical glow discharge in the gas.
Generally at least one member of each dimer is formed from an atom of a rare gas--xenon, argon and krypton have all been used--and the other may be formed from an atom of a halogen gas. Thus diatomic xenon, Xe.sub.2, might be employed in an excimer laser; however, particularly useful wavelengths and other operating characteristics arise from use of rare-gas-halogen excimers, especially rare-gas monohalides such as xenon chloride, XeCl, krypton fluoride, KrF, and to a lesser degree argon and xenon fluoride--ArF and XeF.
More specifically, ultraviolet and even vacuum-ultraviolet laser pulses can be derived from rare-gas-halogen species. The energy transitions of interest generally are between strongly attracting bound energy states and strongly repelling unbound ground states.
Excimer lasers typically have very high gain and are virtually superradiant. In consequence, laser action can be sustained with a fully-reflecting rear mirror and an output window whose reflectivity is only a few (e.g., four) percent.
With excimers, however, "sustained" means maintained for time periods on the order of excited-state lifetimes that are only--for XeCl as an example--ten nanoseconds. Although short pulse duration is desirable in many applications, such an extremely short duration also has drawbacks.
In particular, it creates an extreme requirement for properly timed and uniform electrical discharges, to produce and pump the desired upper states with adequate synchronism and uniformity throughout the laser chamber. Otherwise the resulting laser pulses are relatively very weak.
Accordingly, various strategies are used in attempts to provide a relatively uniform preionization discharge in the laser cavity, before the main discharge starts. Generally if the preionization is relatively uniform then the main discharge will also be relatively uniform.
Thus excimer lasers require relatively expensive power supplies and pulse-forming networks. Electrodes and basic cavity designs too are in general much more expensive than in lasers of other types.
Prior excimer lasers may be considered in two separate categories: transversely discharged units and certain others that have appeared commercially; and longitudinally discharged devices, remaining almost entirely in the lab-research realm.
Commercial units--transversely discharged or microwave-excited: Until now, to take full advantage of the inherent advantages mentioned above, nearly all excimer systems available commercially have been very high-powered. Pulse energy varies from the high millijoules to more than a joule, and repetition rates extend into the kilohertz region; so that average power in some commercial units is in the kilowatts.
Almost all these lasers have been transversely avalanche-discharged. Operation of these devices is similar to that of earlier and more familiar carbon-dioxide "TEA" or transversely excited atmospheric-discharge lasers. Some early excimer lasers were constructed using converted TEA lasers.
These devices represent superb technological accomplishment and are extremely useful tools. They also, however, have certain important disadvantages.
They require very fast coupling of high electrical energy into the laser head, which makes it very critical to assure the long life of electrical components as well as that of the laser proper. This requirement generally leads both to expensive components and to sophisticated, costly protective circuitry. PA1 These lasers have very large discharge volumes, which aggravate the above-mentioned difficulty of achieving uniform preionization and sufficient initial ion density for a uniform glow discharge. This difficulty too is reflected in the added costs of sometimes-exotic subsystems directed to effective preionization. PA1 Poor beam quality (high beam divergence, irregular cross-section, and beam nonuniformity) calls for very expensive and complex optical delivery trains to refine it for use. PA1 Relatively low spatial coherence (probably arising from imperfect synchronicity of lasing in different portions of the cavity) gives rise both to freedom from speckle and to high beam divergence. The former, however, usually is desirable, whereas high divergence is problematic. PA1 Energy per pulse falls relatively quickly once a gas aliquot is placed in a system. This loss results from chemical reaction of the halogen gas with materials of construction of laser components that are exposed to the gas--progressively reducing halogen concentration available for excimer formation and lasing action, and furthermore creating impurities (both gaseous species and solid deposits on windows) that attenuate the laser radiation. These losses necessitate elaborate on-line equipment for gas circulation, purification and replenishment. Cost of the halogens themselves--and managing of gas supplies and exhausts--in many cases are significant.
As a result of these factors, most commercial excimer lasers are very large (and thus inconvenient and expensive to house), and very time consuming and costly in service and maintenance. Above all, they are extremely expensive to buy and to supply with the needed gases.
These costs are particularly troublesome because in many applications--for example, spectroscopy, detecting, and especially semiconductor integrated-circuit failure analysis--usage is typically only occasional or at most intermittent.
Thus a topic of very great importance for manufacturers, vendors and users of transverse-discharge excimer lasers is "gas lifetime". Gas lifetime is often taken as the number of output pulses that can be obtained before the energy per pulse falls to half of the original value. As will be seen shortly, this conventional focus on the number of pulses is drastically different from the thrust of our interest in gas lifetimes.
Various excimer species have correspondingly various gas lifetimes. Hence there is also an important focus upon the gas species chosen, as to the resulting lifetimes.
For example a recent technical note, Laser FAX No. 3 from Lumonics Ltd. (a British manufacturer of excimer lasers, and of sophisticated cryogenic processors for excimer-laser fill gas), includes this observation:
The XeCl transition at 308 nm is by far the most favourable in terms of gas lifetime. Typically, an excimer laser can be operated for 10 to 20 million shots on a single gas fill without any cryogenic gas processor, and the cost of a cryogenic processor is rarely justified for XeCl operation. PA0 "[G] as consumption for the 260 hour test averaged about 35 cents per hour. Test data suggested that discharge tube replacement was desirable after about 300 hours of 2 kHz operation. . . . A new tube with mount can be purchased for $1380, or PPI can replace only the tube at a cost of $590." PA0 "explained assuming that the discharge develops in two stages. First a discharge loads the capacitance consisting of the inner tube wall and the aluminium foil, then after a certain length of time, depending on the discharge parameters, breakdown between the internal electrodes takes place." PA0 (1) in the first form, but not the second, the presence of a gas pump (or a valve leading to a pump) is expressly excluded; thus the second form may include a pump (or a valve that leads to a pump); and PA0 (2) in the second form, the enclosure expressly must be sealed off by a substantially permanent type of seal, whereby it can be reliably handled and transported without compromising its useful life.
The Lumonics publication goes on to show the number of hours of operation for XeCl in a particular Lumonics unit, the Model "Excimer-600"--at 300 Hz with no cryogenic gas processor--as 4.6 to 22 hours. Although the technical note omits to say so, these operational hours must all occur on an essentially consecutive basis, because the gas-degradation rates of these high-powered devices proceed rapidly even when the devices are turned off. Thus the number of hours of operation may be viewed very nearly as calendar time.
The reasons for a high rate of halogen disappearance from commercial transverse-discharge excimer lasers--both in use and essentially on the shelf--arise intimately from the size and nature of the discharge chambers, and the character of the halogen gases preferred for excimer-laser operation. More specifically, chlorine and the other halogen gases are corrosive in the extreme, and react rapidly and even violently with most of the materials of construction of these large lasers.
Those materials include large surfaces of stainless steel, nickel, and polymers such as those known in commerce by their registered trademarks Teflon and Viton. All these materials are exposed not only to the reactive gases, but in almost all cases directly to the electrical discharges formed in these gases for creation and pumping of the necessary excimer species.
The elevated temperatures and ion densities of these discharges produce maximum interaction between the gases and the constructional materials. Furthermore, contamination and degradation of both the gases and the materials during discharge operation may possibly set the stage for still further chemical action later, while the lasers are turned off.
Some manufacturers have introduced protective coatings that purportedly are effective in resisting such reactions. Where these materials are exposed directly to the discharge volume, we consider their effectiveness limited. In any event, as will be seen, all such materials are far more vulnerable to reaction than the materials used in our system.
These reasons for swift deterioration of both the discharge chambers and the operating gases are known. Rather than dealing with them, however, it has become customary in the industry simply to treat them as if they were merely necessary concomitants of the process. Thus most literature in the field sweeps the problem under the rug by referring to "consumption" of the "fuel gases".
Of course it is perfectly well known that, with respect to the energy-pumping and -emitting processes of the laser itself, the gases are all but infinitely reusable; and that the gases are not "consumed," in the manner of "fuel," by the lasing process. Rather, as mentioned above, they are lost to chemical reactions that bind them, in useless chemical combinations, to the discharge-chamber walls and other components--and in both gaseous and solid species that attenuate the laser radiation. They can also be bound in other gaseous species that are inaccessible to the laser mechanisms.
Yet the industry has made little or no movement toward moderating this loss. The reasons for maintaining this expensive and wasteful operating mode are also clear.
First, the high power levels that are favored--to take fullest advantage of the high photon energy and other benefits of the excimer laser--require massive discharge chambers, high voltages and currents, and heavy-duty electrodes.
Thus chambers are typically a few tens of centimeters transversely, and roughly a meter long, with multiple feed-throughs and sturdy internal structures. Electrodes for forming primary transverse discharges are typically continuous solid metal bars mounted to the interior wall, and running the length of the chamber interior, along opposing walls--for example, top and bottom.
Further, some units also have preionization electrodes at opposing sides, sometimes in the form of a row of spark gaps, electrically in series. In other devices instead the main-discharge electrodes are of mesh, with the preionization electrodes behind them.
Now, all such industrial-size apparatuses necessarily entail use of typical industrial materials--steel, plastic, and the like. More-fragile materials usually associated with small laboratory instrumentation, though known to be preferable in terms of chemical inertness, would be impossible or very difficult to use in fabricating such large chambers that must be shipped and installed on a fairly routine basis.
Moreover, even the relatively inert materials such as glass would be difficult to fully clean and render passive in such large industrial forms.
Thus skilled workers in the industrial field of transversely excited excimer lasers think in terms of "gas lifetimes" of days, or at the very most weeks. They also think in terms of continuous, or at least frequent, gas supply to such lasers--with on-line tanks never more than a valve away, and with elaborate, expensive traps and other reprocessing equipment.
As can be seen, this branch of the excimer-laser field spares little in terms of material or money. It applies almost a brute-force approach to attainment of extremely high-energy, high-power laser pulses--for those applications that cannot do with less.
One exception to some of these statements is the product line of Potomac Photonics, Inc. (PPI), of Lanham, Md. That firm offers very compact devices, excited by microwave radiation without electrodes.
Absence of electrodes from the cavity interior makes possible a great reduction in mutual contamination between the laser gases and the materials of construction. In principle this should greatly extend gas lifetimes.
Unfortunately, however, the PPI systems retain many of the adverse characteristics of the large, transverse-discharge systems that have electrodes. First, the system employs a trickle-through gas system, with a pressurized "premix cylinder", a regulator and an exhaust port. In addition the system has a so-called "optional" halogen trap.
Second, gas and component lifetimes are reported in terms of pulses and "continuous operating hours". More specifically, recent PPI literature on these commercial devices asserts:
The noted 300 hours of continuous operation corresponds, of course, to less than two weeks; and the $1380 or $590 replacement cost amounts to an added $4.60 or $1.97 per hour, respectively, beyond the 35 cent-per-hour direct cost of gases. Furthermore there is no indication of the probable lifetime of the tube under the more-interesting assumption of intermittent or occasional use.
Perhaps the lowest-priced excimer lasers on the market, the PPI systems are significant developments. Their distinct operating inconveniences and apparently frequent replacement requirement, however, do leave much to be desired.
Longitudinally-discharged research units: During about the last thirteen years some work has been done on longitudinally discharged excimer lasers. Generally speaking, the advantages of this type of excimer laser are (1) simplicity of the discharge-assembly structure, (2) compactness--i.e., much smaller size than the transverse systems, (3) circular beams with low divergence, and (4) theoretically greater amenability to establishment of a uniform glow discharge, because of smaller resonant volume and relatively lower pressure.
On the other hand, longitudinal systems heretofore have been bypassed commercially because of their much lower energy output. Virtually none of the known systems has moved beyond the basic-research stage.
None of the research reports which we have studied includes consideration of laser lifetime, reproducible or consistent pulse energy, or particularly the quality or effectiveness of preionization. Following is a review of research papers in this area.
In this review we shall emphasize two main questions that relate to our invention: (1) What overall general suggestion do the workers in this field offer as to improvement of apparatus lifetime? (2) What did they understand and teach as to preionization?
An early report is due to Isakov et al., in "Excitation of a XeF laser with a longitudinal electrical discharge," Soviet Tech. Phys. Letters 3 (September 1977), at 397-98. Isakov recounts obtaining 0.8 kW/cm.sup.3 maximum excimertransition power in a pulse that appears to have been about twenty-five nanoseconds long.
Based upon these figures, his output-pulse energy density was apparently some twenty microjoules per cubic centimeter of the discharge tube. Isakov reports using capillaries thirty centimeters long and one to four millimeters in diameter; thus if his energy-density figures apply throughout the capillary size range, his pulse energy must have varied from about 23 to about 380 microjoules.
The last-mentioned figure at first seems quite respectable, considering the apparatus involved. Although using a power supply capable of 500 Hz, however, Isakov was not able to obtain more than one "shot" (pulse) of excimer radiation without replenishing the gas in his device; and even with a continuous-flow system could not turn over the gas fast enough to operate his system beyond 10 Hz.
Isakov was probably correct in attributing the one-shot character of his apparatus to "rapid loss of the NF.sub.2 radical formed in the discharge due to an interaction with the chamber material." He said nothing, however, about pulse-power reproducibility, and expressed no appreciation for the possible application of preionization techniques.
Isakov did not suggest how any of his results might be used to guide development of the excimer laser into a practical tool for routine purposes. Plainly his device was not intended as anything other than a research unit.
Isakov was followed by Burkhard et al., in a technical paper entitled "XeF excimer laser pumped in a longitudinal low-pressure discharge," Appl. Phys. Lett. 39 (Jul. 1, 1981), 19-20. These workers at the Institute of Applied Physics in Bern were able to operate their device with a "stable output" at one hertz for more than a half hour, and obtained a circular (apparently as distinguished from annular) beam.
They were also sophisticated enough to provide a "HF prepulse"--presumably meaning a high-frequency preionization pulse, applied to the same longitudinally arranged electrodes as the main discharge excitation. The cost of these advances, however, was apparently severe in terms of output pulse energy: the reported mean for optimum gas conditions was only 0.6 microjoule.
Burkhard noted that a "significant decrease in the output was observed during the first pulses of a new gas filling". From this observation he drew the conclusion that "an enhancement of laser efficiency should be possible with a gas flow system."
Thus the first and major teaching of a pioneering worker in regard to gas lifetime of longitudinal excimer discharges seems to have been an explicit suggestion to follow the lead of the transversely excited systems. Accepting that solution would start this field upward on the same complexity-and-cost spiral toward progressively more elaborate systems.
Burkhard also suggests that a desirable direction for further attention might be to increase the full beam angle of his apparatus "by using a resonator with discrimination of higher-order transversal modes."
Closely following Burhard, in turn, is a research paper of Cleschinsky et al., "XeF-Laser with Longitudinal Discharge Excitation," Optics Communications 39 Sep. 15, 1981), 79-82. The apparatus used in that work included a complete gas-handling system, including a cold trap, pump, pressure-control system, and isolation valve.
In addition, the discharge system isolated by that valve includes a reservoir "to increase the operating period of a gas filling." The reservoir is in a tubulation loop with the discharge cavity; and a diaphragm pump is also installed in this same loop to circulate the gas through the loop and thus through the discharge chamber.
The Cleschinsky system was "operated without preionization". This comment reveals, of course, that Cleschinsky was aware of the use of preionization techniques, but presumably considered them inappropriate for his purposes.
Encircling Cleschinsky's discharge tube was a foil wrapping, connected to his ground electrode; this foil wrapping apparently was not used in any way which Cleschinsky considered preionization. Rather, he viewed it simply a part of his longitudinal main-discharge system, and in his words it was included "to improve the reproducibility of the discharge".
In his report, Cleschinsky describes attainment of 300-microjoule output pulses, and a gas lifetime of a thousand pulses per filling--or 300,000 for the system if the gas is replenished. Hence the teaching of these leading workers, once again, is to follow the lead of the transversely excited systems in rating or valuing performance in terms of the number of pulses.
Cleschinksy and his colleagues in the Optical Institute at the Technical University of Berlin have since reported on closely related experiments--apparently with substantially the same apparatus--in Eichler et al., "KrF laser with longitudinal discharge excitation" Appl. Phys. Lett, 46 (May 15, 1985), 911-13; and again more recently in de la Rosa et al., "ArF laser excited in a capacitively coupled discharge tube", J. Appl. Phys. 64 (Aug. 1, 1988), 1598-99.
Eichler, emulating his colleague Cleschnisky, used a grounded capacitive foil-wrap cathode, and said it was to "improve the reproducibility of the laser discharge." He also "found [it] to increase the output power."
De la Rosa, however, used no internal cathode and hence no glow discharge. Instead the discharge was struck to a capacitively coupled foil only, and was exclusively corona.
The Eichler paper, addressing krypton-fluoride operation, reports pulse energy of only fifty microjoules at two hertz; while the de la Rosa installment, in relation to argon fluoride, describes pulse power even lower--fifteen microjoules, but at ten hertz. Neither reports any gas-lifetime data.
Although this series of papers progresses nearly to the time of the present writing, none of the reports expresses any interest in preionization or proposes that such techniques might have any value for practical devices or otherwise. To the contrary, Cleschinsky's original paper concludes "further improvement seems possible by optimizing the driving circuit."
Similarly, Eichler proposes, "Further improvement should be possible using higher excitation voltages"; while de la Rosa predicts, "Energies up to 100 .mu.J should be possible using low-loss mirrors." It will be understood that during the course of this series of reports from Berlin, preionization meanwhile in the transverse-excitation branch of this field has achieved substantially general acceptance.
Following the initial report of Cleschinsky from Berlin, one of the present inventors reported from the Shanghai Institute of Laser Technology. The paper is Zhou et al., "XeCl excimer laser excited by longitudinal discharge", Appl. Phys. Lett. 43 (Aug. 15, 1983), 347-49. In the related work, no reservoir was used.
The paper reports maximum pulse energy of 317 microjoules in a glass capillary, but no average-power or gas-lifetime data. That value of energy, moreover, was not obtained frequently; in fact, the stability or reproducibility of pulse energy from shot to shot was far below the quality that would be expected of a commercial system.
Further, the apparatus was not operated continuously at all, but only on the basis of one and two shots at a time. The total number of shots over the entire project was about 10,000--after which the pulse energy was generally at about seventy percent of the original level. During these experiments, gas was replenished two to three times a week, through a valve which remained in line between the laser and the gas-handling system.
The report from the Shanghai Institute was the first to describe essentially d. c. preionization in a longitudinal-main-discharge system. Preionization was provided by a corona discharge through the capillary wall, between a foil wrapping and the anode.
Thus the preionization geometry was essentially longitudinal--from the anode to a point along the tube at which the external foil wrapping was effectively coupled capacitively. This latter point initially was adjacent the end of the foil wrapping closest to the anode, but later shifted progressively along the interior of the foil--so that the preionization discharge itself propagated lengthwise to nearly the full length of the tube.
In disagreement with the proposals of the Bern and Berlin groups--to seek improvement through use of a gas-flow system or "discrimination of higher-order transversal modes" (Burkhard), a refined "driving circuit" (Cleschinsky), higher voltage (Eichler), or "low-loss mirrors" (de la Rosa)--the Shanghai researchers' paper proposes that solutions lie in the direction of "preionization to improve the discharge uniformity" and in matching the impedance of the transmission line from the driving circuit to that of the laser.
The Eichler 1984 and de la Rosa 1988 analyses follow the appearance of Zhou's 1983 report from Shanghai. Thus the latter was not accorded general acceptance, by those skilled in the art, as definitive of the problem or its solutions. Rather, the analysis of Zhou has simply remained one in a group of broadly divergent teachings.
Careful comparison of the first two Berlin papers with Zhou's article, however, reveals surprisingly that experimental arrangements were very similar. In fact both Cleschinsky and Eichler employed the same grounded-foil-wrap system which gave Zhou "preionization".
Evidently the Berlin group, while studying, analyzing and describing the laser output in detail, did not appreciate the physical significance of the capacitively coupled foil in preionization terms. Thus they could not fully recognize either why it aided output energy and reproducibility, or how it might be improved.
Cleschinsky, in fact, insisted that his device operated without preionization; but his colleague Eichler, apparently carrying on the same work, observed some phenomena that he
It is at least plausible to speculate from this that a longitudinally propagating corona provided preionization in the hardware used by Eichler (and Cleschinksy) too. Although Zhou was able to see preionization as a valuable function, at the time he overlooked the limitations of the longitudinally propagating preionization system that he was using.
Meanwhile Dutch researchers were working with a longitudinal tube that was capacitively coupled from end-plug anodes to a foil-wrap central cathode. It is described in Gerber, "A KrF-Laser Excited by a Capacitively Coupled Longitudinal Discharge," Optics Communications 53 (Apr. 15, 1985), 401-04.
Gerber describes a longitudinally propagating discharge that appeared in his tube. That discharge apparently was very similar to the longitudinally propagating preionization corona in the Zhou paper (and possibly the two Berlin papers) just discussed. Although Gerber attained pulse energy of 900 microjoules, his beam was ring-shaped or annular, and he reported no data for average power, repetition rate or life.
At first Gerber operated his apparatus with the cathode capacitively coupled (like de la Rosa's). Later, however, Gerber converted one of the end-plug anodes to a cathode by connecting it to the foil cathode.
When operating this modified system at pressures below one bar, he obtained a directly coupled longitudinal discharge between the two end-plug anodes--thereby creating a system very much like those of Cleschinsky, Eichler and Zhou. Nevertheless Gerber achieved no increase in output energy, pulse duration, or even uniformity of the beam cross-section, which remained annular.
Although he was able to use his capacitive discharge as a preionization pulse for a direct longitudinal main discharge, Gerber--like the three researchers using similar systems before him--failed to discern what it was about his system that precluded fully acceptable main-discharge properties.
He concluded instead that "this discharge mode is not suited to . . . KrF molecules." Gerber's only suggestion for improvement of the apparatus is that "still higher pulse energies could be obtained by a further increase of the applied voltage."
In the next year (still before the paper of de la Rosa), Furuhashi et al. reported from Nagoya, Japan, on yet another divergent approach. Furuhashi's group used preionization by spark-generated ultraviolet light.
The discharge tube in this work consisted of Teflon tube segments, four millimeters in diameter, interconnected by nylon or Teflon fittings. Spaced at six-centimeter intervals along this structure, Furuhashi installed five pairs of stainless-steel electrodes, each pair forming a four-millimeter spark gap.
In the resulting report, "Longitudinal discharge XeCl excimer laser with automatic UV preionization," Appl. Phys. Lett. 50 (Apr. 6, 1987), 883-85, Furuhashi conceded that his "laser output energy is not very large, around 100 .mu.J, and also the efficiency is not very high". Perhaps even more significant is the absence of data on repetition rates, and the observation that the number of shots obtained without gas replenishment was only "over 1000".
Furuhashi's proposed cures, however, went off in yet other directions, different from those of the other skilled workers in this area. He proposed to improve performance by optimizing "various parameters such as the mirror reflectance, the tube diameter, and the length of segments . . . and using a gas flow system" (emphasis added).
In this last-mentioned regard, Furuhashi joined Burkhard in teaching that a primary requirement for advance in longitudinal-discharge excimers must be a gas-flow system. Thus for Furuhashi and Burkhard a primary thrust of this field should be to follow the lead of the big transverse-discharge systems.
Overview of the prior art: Transversely discharged high-power excimer systems have been commercially successful but are large, require special installations to accommodate halogen-gas management, and are extremely expensive in both acquisition and maintenance. Their power output is far higher than needed by many users.
Some of the lab-research papers discussed above have mentioned the idea of adapting the longitudinal excimer laser for use in a group of low-power applications. Gerber and Zhou, for example, referred to "miniature systems [for] e.g. spectroscopy and testing" or "pumping of dye lasers and the first step pumping in amplifying the transversely excited excimer lasers, etc."
From their technical writings at the same time, however, it is clear that these comments were only made as abstractions and that none of the researchers knew how to go about actually making such systems. As will be seen in a later part of the present document, at least one of those early experimentalists was far from any practical development.
Hence there has been heretofore no practical connection between the very large, high-powered transversely discharged devices and the longitudinally discharged experimental units. The only suggestions from researchers for improvement of the latter have been very helter-skelter. As recounted above, they range from driving-circuit refinements, or "mode control", to greater use of gas-flow systems.
This last-mentioned one of the disparate proposed directions--namely incorporating a gas-flow system--is closely related to one of the two topics mentioned earlier as being of main concern in this document: gas lifetime. The approach of using gas-flow systems is noticeably more popular than others.
This approach is apparently also endorsed by the designers at PPI. The Potomac Photonics instrument, being microwave excited, is not readily characterized as discharged either transversely or longitudinally. The designers of that device are virtually the only writers to propose that a practical excimer discharge tube should and could be made of material minimally reactive with halogen gas.
Even those writers, however, have moved in the direction of the flow-through gas system. PPI's approach accordingly is to evaluate gas lifetimes in terms of pulses or operating hours, rather than shelf time.
In adopting that solution as the principal thrust for exploiting longitudinal-discharge excimer lasers, the longitudinal-discharge field would follow the developmental course of the transverse-discharge systems--whose drawbacks we have already pointed out. This predilection of the researchers for such a system is understandable in context.
First, all known commercial systems employ elaborate gas-handling systems, most using some form of gas-flow system. Secondly, all the researchers themselves very apparently worked with gas-handling systems either continuously attached or at the ready--that is, just at the other side of a valve from the discharge system.
In a research context such facilities are not particularly extraordinary or troublesome. Researchers' preference for flow systems is therefore understandable.
The gas-flow operational mode, however, is very undesirable for exportation into the industrial context. There, personnel may not be as consistently well informed, careful and patient in the management and handling of highly reactive gases; and the pressing importance of laboratory-throughput or production achievements may be more dominant.
Consequently management of corrosive gas supplies--and proper exhaust of waste gases--in many facilities can pose not only an inconvenience but also significant problems of safety. This is particularly true when a laser is used only occasionally, affording inadequate opportunity to develop proper procedures into good habits.
The use of systems with short shelf life also places a nearly impossible burden on facilities managers to plan ahead for use. In many cases--such as, for example, integrated-circuit failure analysis--demand for use is unpredictable.
Even if an apparatus does happen to be ready for use in terms of its shelf life, considerable preparation is needed for beginning operation of the laser. In terms of the desired use, such preparation represents lost time.
Gas flushing may be required. Manipulation, adjustment and stabilization of gas parameters may be touchy. The cost of the gas itself, both in these preliminaries and in laser operation, can be significant.
Furthermore, using a flow-through laser system with accompanying gas-supply and gas-handling equipment tends to militate in favor of giving the laser system itself the status of a facility. That is, it has its own stationary plumbing and other installation requirements.
Application apparatus to be used with such a system must accordingly be moved to the facility site of the laser. This can be a significant drawback, for often it is preferable to disturb the application apparatus as little as possible--by taking the laser to the application.
Turning now to the topic of preionization, none of the researchers specifically pinpointed it as an area importantly needing refinement. In all the reports discussed here, preionization is either ignored or employed in a clearly ineffectual way--at least by the standards of commercial-quality instrumentation.
In the experimentalists' papers discussed here, at least taken as a whole, no clear theory is stated for the importance of preionization, especially in longitudinal lasers. There is surely no plain statement in these works of (1) what constitutes good preionization or (2) how to go about achieving it.
This comment is true even through it may be remarkable in view of the familarity of preionization in transverse systems. The longitudinally propagating preionization pulse apparently employed--with varying degrees of awareness--by Cleschinsky, Eichler, Zhou and Gerber, is subject to deficiencies that can be plainly articulated. The same is true of the spark-ultraviolet system of Furuhashi.
Specific identification of the preionization deficiencies in these systems, and of a proper course for effective preionization, is part of the making of the present invention. Accordingly it will be presented in a later section of this document.